A question of loading...


I have just replaced my aging tube preamp with a new model. When I was using my older model with tube phono stage, I would run my Lyra Kleos and other MC cartridges with a load of 750 ohms. So, I assumed that with my new tube phono stage, which also utilizes a transformer in the phono stage and is also built into my new preamp, that the same cartridge loading would apply. I listened to the Kleos for about a week with this loading, and frankly, while it was fine, I wasn’t bowled over. Tonight I decided to experiment, so the first thing I did was to run the cartridge straight in, with no loading plugs. WOW, the increase in overall musicality and soundstage width was eye opening! Lesson learned is that not all gear is going to react the same when it comes to cartridge loading, particularly if there is a transformer involved and even if you are using the same cartridge from one phono stage to the next! An eye opener, anyone else experience something like this?
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OTOH, do tubes have a distinct advantage?? I would say that they do when it comes to overall SQ, plus the decreased likelihood of overload, BUT they are never 100% quiet...at least IME. There is always some minimal hiss..and this leads me to believe that their noise floor has to be higher than ss.
Yes, they are noisier but they can be quiet enough that 0.2mV is no worries.
Jonathan Carr never says anything remotely similar about optimum loading for his cartridges. I am looking at Jonathan Carr's loading chart that came with my Etna as I type this and depending upon the gain setting and the total capacitance of the tonearm wire in interaction with the phono stage and the recommended loading ranges between 104 and 340 ohms for 0dB gain and between 284 and 887 in the highest gain column of +6dB.
But that is Jonathan Carr who sees things (rightly so) from the perspective of the cartridge manufacturer. Now lets talk about phono stage producers. They don't espouse charts, they talk about listening. Each phono stage manufacturer knows the attributes of their designs. My Manley Steelhead with built in step-up transformers is not going to behave the way a Sutherland 20-20 (just for example) will.
To say that 47K is the standard or de facto or even starting-point loading for a LOMC is ludicrous. 
@fsonicsmith 

You might think it ludicrous, but 47K is the industry standard for phonograph inputs nonetheless. Because many phono sections have problems with the RFI generated by LOMC cartridges (keeping in mind how much gain they employ to work with cartridges of output this low) its common to see loading options on such preamps. Think about it this way: Since all cartridges are different, and you need an input resistance with any circuit that can amplify, what would be the correct value for that resistance?? You might ask your self why most phono sections have a 47K input impedance... the answer is that is the industry standard; it was not something that is a random coincidence that so many phono sections are built this way.


Jonathon has his recommendations for a very simple reason: He cannot be sure that the phono section to which you are connecting his product is going to be alright with the RFI generated by the cartridge/tonearm cable combination. Further, he's also go not idea what cable you're using! So he has loading recommendations on that account, and they cover a range rather than being a specific value. He also knows that most phono sections don't deal with the RFI issue very well. But if you talk to him, which I have done in person, you find out that he thinks 'no loading' (IOW the stock 47K input of the phono section) is better if you can pull it off.

Its true as you say that not all phono sections act the same :)  That is a bit of an understatement! For example, a phono section employing an SUT will not have RFI issues, since the RFI is blocked by the SUT. Instead, you have to be careful to load the output of the SUT correctly so that the transformer does not 'ring' (distort) on account of the specific impedance of the cartridge (if you want to know more about how the ringing phenomena works please ask). So you'll find that depending on the cartridge, different resistor/capacitor values are used to accomplish that loading. So yeah, that's quite a bit different from a solid state phono section using opamps or an all-tube phono section that can run the LOMC cartridge straight in!

There is no single advantage, today SS phono stages just can't be overloaded in normal condition: comes with very good headroom. That " problem " was a " problem " of the past/several years ago.
With a solid state phono section using either opamps or discreet transistors, its the part that is outside the feedback loop that is open to overload. This is the input circuit of the preamp, which might be no more than the base of the input transistor.  We're talking about an electrical peak of 20-30dB that occurs with all LOMC cartridges in tandem with the tonearm cable (the former having a high-Q inductance, the latter having a capacitance; the two in parallel form the resonant circuit, whose resonance might be at several MHz). While there are SS phono sections that deal with this properly (Pass Labs for example) its a falsehood to say that all of them do!


Its a simple fact that if the designer has not made provision for this resonant peak, it can overload the input circuit, resulting in a tick or pop when it does so. To get around this problem, you have two options: design the phono section to take quite a bit more input voltage than LOMC cartridges are known to produce (and do as much as you can to prevent RFI from coming in through the input connectors), or come up with a loading provision (the 'loading resistor'), so the resonant peak can be detuned.


Of course if you use the loading resistor option, you are asking the cartridge to do more work. If you're using 100 ohms as a loading resistor, that's a couple of orders of magnitude more work than if the cartridge is driving 47K. This results in the cantilever being stiffer- the same as what happens with a raw woofer if you short it out (they are both based on the same principle of operation).  This is not a matter of debate, if you feel the desire to do so, take it up with Mr. Ohm. Ohm's Law cannot be defeated and isn't open to interpretation. If there is more current flowing, it has to come from somewhere and that somewhere is the motion of the stylus in the groove. So the fact that the cantilever gets stiffer is not controversial. Anyone versed in the art knows this- Jonathan Carr and I discussed this issue at the Munich Show a few years ago. 


Raul's stipulation that this overload issue is something of the past is correct as most phono sections were only designed to have 'enough gain, low enough noise and proper EQ' and the electrical resonance was ignored. But he is incorrect in his assertion that this has been put to bed; this is why loading provisions exist on current equipment and are identified as 'loading' when they are really there for the benefit of the phono section, and the phono section will sound different if the RFI isn't suppressed. Its likely that the cartridge will sound different too, not because you are preventing it from ringing, but because the cantilever becomes stiffer as the 'load' resistance is decreased. This measurably affects its mechanical resonance in the tonearm. 





fsonic, May I just ask a question without making you angry or making anyone else angry.  As the owner of a Manley Steelhead myself, I am wondering how you achieve a load of 50K ohms or 100K ohms or "25", by which I take it you mean 25 ohms?  As you know, the max load if you use one of the MC inputs on the Steelhead is 400 ohms, and it offers choices going down from there to 25 ohms minimum.  I assume that is what you did in that case.  The Lyra website says the Etna Lambda has 4.2 ohms internal impedance, so 25 ohms would be a bit on the low side for phono input resistance and your description of the aural effect makes a lot of sense in that context.   The Steelhead uses autoformers in its MC section to achieve the various load resistances, which is a bit different electronically from using a SUT. I'm sure you know that.  I sure wish I could get hold of a schematic, because I would like to see how they use the autoformers.

If you connect an MC cartridge to the MM input, there you can have 47K ohms, but 50K and 100K are not offered.  However, if you change the fixed load resistor inside the chassis from 47K to 50K or 100K, by that means only can you achieve either of those two load resistances.  Is that what you have done?  I've actually purchased the resistors to change the max MM load resistance from 47K to 100K, but so far I have not done it.  I found the Steelhead to benefit greatly from a modification to its output stages (both the phono output and the passive linestage output), for what it's worth.

Not that it matters a lot, but I have to agree with Dover. In the conventional parlance, to increase the value of the load resistor is to decrease its load.  Capacitance is also a "load", but that is a different parameter.  In any case, I knew what you meant, when you discussed the effects of various load resistances, and that is what counts.
@lewm 
Not angry at all-in fact Lewm-you are among a handful of regulars on this Board that I rely upon on all analog matters and respect as having greater grasp of technical issues than I do. 
I previously acknowledged to Dover that he was correct and that I had misspoke as to loading. 
As you know, the max load if you use one of the MC inputs on the Steelhead is 400 ohms, and it offers choices going down from there to 25 ohms minimum. I assume that is what you did in that case. The Lyra website says the Etna Lambda has 4.2 ohms internal impedance, so 25 ohms would be a bit on the low side for phono input resistance and your description of the aural effect makes a lot of sense in that context.  
Yes, I meant that 25 ohms on the Steelhead with the Etna (and my VdH Crimson before it) sounded loose and sloppy, 50 ohms sounds ok on the Etna and sounded best with my VdH Crimson, and 100 sounds best with the Etna Lambda in terms of sounding tight and controlled without all the life being sucked out of the music, which occurs at all higher levels. The lower settings are euphonic but distorted. And yes, I use the two MC inputs only and not the MM. And I knew and agree that my use of the term "internal step ups" was sloppy and loose too (pun) as Manley refers to them as both in the owners manual (though I may be wrong and maybe they use the term "autoformer" exclusively) but as you stated, without seeing a schematic, even an informed person like you would find it difficult to explain what is going on and why. 

Ralph-thank you for your very polite and informative response. I apologize for my strong words directed at you. 
@atmasphere Ralph, thank you for contributing to this topic. What you posted was extremely informative and interesting. I certainly think that a lot of fellow a’philes can learn a lot from you.

In my case, i am still a little surprised as to why the new preamp which features a phono overload spec of 70mv rms for MC’s is so very different to my old model, I am guessing that this is high enough to not risk ringing with my particular cartridge, a Lyra Kleos that puts out 0.5mv. What is odd, and I am not quite understanding this part, is why with my older CAT phono stage, which did not use a SUT at all, that a load of 750 ohms was better sounding than straight in at 47Kohms, yet with my new model, this is definitely not the case. Presumably due to the SUT in my new CAT?? One thing, the CAT has a healthy output voltage at about 50 volts..